Silver ions and silver compounds are recognized to have a toxic effect on some bacteria, viruses, algae and fungi that is typical for heavy metals like lead or mercury, but without the toxicity to humans that is typically associated with heavy metals like these. For example, silver has been shown to kill many microbial organisms in vitro (i.e., in a test tube or a petri dish). The antimicrobial properties of silver are believed to be due to an oligodynamic effect, in which silver ions denature proteins (e.g., enzymes) of the target cell or organism by binding to reactive groups or sites therein, resulting in their precipitation and inactivation. Silver may inactivate enzymes, for example, by reacting with the sulfhydryl groups therein to form silver sulfides. Silver may also react with the amino-, carboxyl-, phosphate-, and imidazole-groups therein to diminish the activities of lactate dehydrogenase and glutathione peroxidase. Bacteria (gram-positive and gram-negative) are in general affected by the oligodynamic effect, but some species can develop a silver-resistance.
In view of the known antimicrobial properties of silver, there has been great interest in using silver in a number of commercial products in order to impart these antimicrobial properties to those products. More recently, this interest has focused on the use of silver in the form of nano-particles, silver colloids, and/or particles of silver salts, in, for example, coatings applied to the surface of consumer products such as cell phones, clothing items (e.g., shirts, socks, insoles and undergarments), toothbrushes and tooth pastes, soaps, shampoos, facial creams, and internal washing drum of clothes washers. These very small particles of silver are believed to be even more effective at repelling or killing bacteria, viruses, etc., that come into contact with these surfaces.
Although silver has a number of advantages, it does have some limitations. For example, silver and silver-containing coatings are known to tarnish, resulting in an unfavorable appearance on the surface of the product to which it is applied. In addition, some silver-containing coatings may have a concentration of nickel therein that is sufficient to cause allergic reactions to the skin of some individuals who come into contact with these silver-coated product surfaces, or may fail to act as a sufficient barrier between the user's skin and nickel present on the component upon which the coating has been applied. Accordingly, a need continues to exist for a silver-containing coating that is more resistant to tarnishing, that is less likely to result in triggering an allergic reaction to the human skin or is a more effective barrier to nickel migration therethrough (i.e., is hypoallergenic), and yet still possesses a high level of antimicrobial effect.